I agree with you about taking passengers to fill the empty seat - I've been trying to give as many people as possible the "Chipmunk Experience" whenever I go flying.
£20,000 for a year's maintenance seems a lot. I presume they either had to have some significant repairs/rectification or a major component replaced (engine, prop etc). If you're interested in talking to the Prestwick Chipmunk operators, their contact details are on their website at
www.skylarkaviation.co.uk - I'm sure they'd be happy to give you some more information.